In recent years joist girder floor and roof systems have become increasingly more popular as a structural system. Joist girders are a manufactured product and serve as a replacement for steel beams. In general, economic benefits will result in the substitution of joist girders for rolled beams in floor and roof systems. Conventional engineering practice is to design the joist girders as simply supported members, i.e. the ends of the joist girders are free to rotate. The design procedure follows design procedures established by the Steel Joist Institute in Standard Specifications for Joist girders adopted by Steel Joist Institute May 15, 1978, Steel Joist Institute, Richmond, Va. Steel joists which support flooring material or roof deck typically rest on the joist girders. The joist girders in turn are typically supported on steel or concrete columns. Typically, no attempt is made to achieve beam continuity by connecting the ends of the joist girders where they meet at a column.
The invention described herein relates to the use of end ties for connecting the adjoining ends of joist girders together, thereby providing continuity between joist girders at the supporting column. The purpose of using end ties is to create a horizontal end force through the ties to significantly reduce the axial forces in the upper and lower chords so that lighter weight chords can be employed to thus reduce the cost and weight of the joist girders required to accommodate the design load.
Continuity between adjoining structural elements and beams has been used for many years. For instance, steel beams are often positioned over the tops of supporting columns in a continuous manner, i.e. joined end-to-end. The use of continuous steel beams as opposed to simple span beams results in the use of smaller sized beams, thus reducing weight and cost. During the late 1950's plastic steel design concepts were developed in order to achieve an even greater economic benefit in continuous beam systems. This design concept is predicated on a material property characteristic of most structural steels. Specifically, the ability of steel to reach a given stress level (yield strength) and then to flow plastically without an increase or decrease in the stress level. The plastic design procedure makes use of this property by recognizing that once a beam reaches yield levels at highly stressed points the steel will "flow" and a redistribution of internal stresses will occur. This redistribution allows the designer to select beams of less weight, which again reduces cost. In addition to the required steel behavior, the steel beam must possess certain geometrical cross-sectional properties in order to permit the mentioned redistribution to occur without premature beam flange of beam web buckling. Should flange or web buckling occur prematurely, then the beam will not reach its full predicted load capacity and an inadequate factor of safety would exist. Most steel beams manufactured in the United States and foreign steel mills have the required geometrical cross-sectional properties to permit plastic design procedure.
Plastic design concepts permit the selection of a beam cross section to be based on an ultimate design moment of (1/16)wL.sup.2 ; where w is the factored load per foot (safety factor times design load), and L is the beam span length. This moment is the optimum moment that can be used in design for a uniformly loaded structural member.
This optimum moment can also be achieved by using cantilever construction systems ("drop in systems"). These procedures have also been used for many years with steel beams and also in some cases with steel girders. Unlike plastic design procedures, this method does not rely upon yielding of the member or the reliance upon redistribution of stresses in the member in order to achieve the optimum moment condition of (1/16)wL.sup.2, but rather by judiciously selecting the length of a cantilever from the support. Currently both the plastic design technique and the cantilever construction method are in common use for steel beams. The Fish U.S. Pat. No. 2,588,225 illustrates the cantilever construction.
Joist girders have not been designed using plastic design procedures because of very special design precautions which must be followed. In 1973, Croucher and I proposed a construction in which plastic design concepts could be used for steel trusses; Croucher and Fisher, AISC Engineering Journal, First Quarter, 1973, Vol. 10, No. 2, l pages 20-32. This concept required fixity of the ends of the trusses to supporting columns, with the yieldable mechanism being the end portions of the upper chord. This was made possible by redesign of the conventional truss diagonal layout. However, since the required geometrical layout and the connection requirements are "non-standard" for fabricated trusses and for steel joist girder fabricators, the procedure is not readily used. Cantilever construction techniques are occasionally used with joist girders and trusses; however, they have not met with wide acceptance due to connection costs and because they do not fit withing standard product lines for joist girder manufacturers.
By means of the present invention, standard joist girder geometrical layouts can be used, with reduced chord sizes as compared to simple spans or fully continuous spans. Load (stress) redistribution can be accomplished as in plastic design of beams without cost penalty for connections or non-standard layout. The tie connection angles or plates can be designed to yield at a predetermined moment so that a maximum moment of (1/16)wL.sup.2 is created. The end result is a significant weight savings in the joist girders without the penalty of high cost field connections or changing existing standard geometrical layouts.
Tie plate connections that yield have been used by designers of multi-story steel frames to connect beams to columns. This concept of "semi-rigid connections" or "wind connections" has been used to provide a given moment capacity at a beam to column joint. The connections are designed to provide a given (determined) moment resistance from the beam to the column. The present invention is not used to transfer moment from a beam to column, but rather to achieve a load transfer across the top of the column, i.e. to transfer moment (force) from joist girder to joist girder.
Other prior art of interest is U.S. Pat. No. 3,793,790. In this patent the object is to reduce the size of the column by using a deflection pad to reduce the column moment caused by deflection of the lower chord of a joist girder under load.